Companion to the IV custom maps history. This deals with the scene's decline instead of its existence.

Three factors

The quality of the port. The GTA IV PC version had poor optimisation for years. Modding became less attractive due to mods compounding instability on top of what was already there.

Audience timing. PC-modding culture was less prominent in 2009 than it would become by 2015 when V released.

Grand Theft Auto V's pull. When GTA V released on PC in 2015, most of the active modding scene moved over. IV's scene diminished by attrition.